Polyvinyl butyral (hereinafter “PVB”) is commonly used in the manufacture of polymer sheets that can be used as interlayers in light-transmitting laminates such as safety glass or polymeric laminates. Safety glass typically refers to a transparent laminate comprising a PVB sheet disposed between two panes of glass. Safety glass often is used to provide a transparent barrier in architectural and automotive openings. Its main function is to absorb energy, such as that caused by a blow from an object, without allowing penetration through the opening. Additives to the sheet formulation generally include at least one adhesion control agent (hereinafter, “ACA”) to modify adhesion of the sheet to the glass so that a suitable level of adhesion can be maintained in order to prevent spalling of the glass while still providing adequate energy absorption if an impact occurs.
The polymer interlayer sheet can also be modified to impart additional desirable characteristics to safety glass. For example, polymer interlayer sheets can be produced to better attenuate acoustic noise, reduce UV and/or IR light transmission, and/or enhance the aesthetic appeal of openings.
Safety glass can be formed by a process in which two layers of glass and a plastic interlayer, such as PVB, are assembled into a pre-press, tacked into a pre-laminate, and finished into an optically clear laminate. The assembly phase can involve laying down a piece of glass, overlaying a PVB sheet on that glass, laying down a second piece of glass on the PVB sheet, and then trimming the excess PVB to the edges of the glass layers.
The plastic interlayer can be produced by mixing PVB polymer with one or more plasticizers, and optionally with one or more other ingredients, and melt processing the mix into sheeting, which typically is collected and rolled for storage and transportation.
During one lamination process for automotive windshields, sections of PVB sheet are cut from the roll and these cut sections are shaped and/or stacked for assembly. A cut section then is taken from the stack and assembled in a layered arrangement with a rigid substrate (for example, a glass sheet having a particular optical quality) such that a face of the rigid substrate and a face of the cut section are disposed in contact with each other to form a pre-press laminate assembly. Alternatively, this laminate assembly can be formed by interleaving multiple cut sections with multiple rigid sheets.
Plasticized PVB sheet, whether in roll form or in stacked form, can tend to stick to itself or “block” at temperatures that can be encountered before and during the laminating process. This blocking is generally not desirable, and can lead to increased production costs.
Conventional attempts to enhance the blocking resistance of PVB include mechanical roughing of the sheet surfaces (for example, embossing), applying a powder such as sodium bicarbonate to the sheet surfaces in contact with each other, and chemically treating the surfaces of the PVB sheeting. Such surface treatments can, however, often create undesirable handling or adhesion properties.
In other conventional methods used to prevent blocking, PVB sheeting can be interleaved with another sheet material, such as polyethylene, or can be stored and transported under refrigeration, for example at temperatures from about 5° C. to about 15° C. However, for some compositions of PVB sheeting, such as PVB sheeting having relatively high plasticizer content, these anti-blocking techniques may not produce optimal results.
Another technique for preventing blocking entails incorporating anti-blocking materials into the PVB. Incorporation of such materials into the PVB, however, can adversely affect the optical properties of the resulting laminate or the adhesive properties of the finished PVB.
Accordingly, further improved compositions and methods are needed to enhance the blocking resistance of polymer sheets, and specifically PVB sheets, without adversely affecting the optical clarity of the finished laminates of the glass or adhesion properties of the resulting PVB sheet.